Ion selective electrodes

ABSTRACT

ELECTRODES FOR DETECTING AND MEASURING THE CONCENTRATION OF THE CATION OF AN ALKALINE EARTH METAL ARE MADE BY MELTING A STEARATE OF THE METAL DOPED WITH THE LIKE STEARATE OF ANOTHER METAL HAVING A DIFFERENT VALENCE AND HAVING AN ION WHOSE IONIC RADIUS IS NEAR TO THAT OF THE ALKALINE EARTH METAL ION. A LARGE CRYSTAL IS THEN GROWN BY A KNOWN SINGLE-PULL ZONE REFINEMENT APPARATUS AND THE CRYSTAL UTILIZED IN A WET ELECTRODE. WHEN THE ELECTRODE IS PLACED IN A SOLUTION CONTAINING THE ION OF THE ALKALINE EARTH METAL USED THE CRYSTAL DISPLAYS A CHANGE IN ELECTRIC POTENTIAL WHICH MAY BE MEASURED.

March 14, 1972 P. P. HO

10N sELEcTIvE ELEcTRoDEs Filed Oct. 15, 1969 OPIIUUOO nUO INVENTOR.

PAULINE P. HO

United States Patent O 3,649,568 ION SELECTIVE ELECTRODES Pauline P. Ho, Syracuse, N.Y., assgnor to Syracuse University Research Corporation, Syracuse, N.Y. Filed Oct. 15, 1969, Ser. No. 866,570 Int. Cl. C11c 1/00; H01b 1/06; H01l 3/00 U.S. Cl. 252-521 2 Claims ABSTRACT OF THE DISCLOSURE Electrodes for detecting and measuring the concentration of the cation of an alkaline earth metal are made by melting a stearate of the metal doped with the like stearate of another metal having a different valence and having an ion Whose ionic radius is near to that of the alkaline earth metal ion. A large crystal is then grown by a known single-pull zone refinement apparatus and the crystal utilized in a Wet electrode. When the electrode is placed in a solution containing the ion of the alkaline earth metal used the crystal displays a change in electric potential which may be measured.

BACKGROUND OF THE INVENTION This invention relates to electrodes for the detecting of the presence in a solution of cations of an alkaline earth metal and the measurement of the concentration of said cations in the solution.

Such electrodes would be useful in measuring water hardness, detection of contaminants in Water and milk, and measuring cations in body fluids but, heretofore, satisfactory electrodes for such tests have been difficult to make and to use.

Work has been done by Harry P. Gregor and Harold Schonhorn, reported in Volume 81 of the Journal of the American Chemical Society, page 3911, under date of Dec. 11, 1958, in making a multilayer membrane electrode of alkaline earth salts of stearic, hexadecyland octadecyl sulfuric and hexadecylorthophosphoric acids. The salt crystal layers have to be skimmed, layer by layer from a solution in which the concentration f the salt, the pressure and the pH have to be carefully regulated and special buffers must be used. Making the socalled wet electrodes in which the multilayer membrane is used also, even if made by one with a high degree of skill, results in a major proportion of imperfec and non-functioning electrodes.

Other experiments with organic crystals comprising anion radical derivatives of 7,7,8,8-tetracyanoquinodimethane (TCNQ), reported in Chemical and Engineering News, Vol. 39, January 9, 1961, page 42, result in crystals having extremely low electrical resistivities but the conduction is quite different from that in the common semiconductors and there is no known way to use these crystals as electrodes.

SUMMARY OF THE INVENTION The present invention contemplates the adding of certain chosen impurities to the alkaline earth salts of longchain fatty acids to make them suitable for use in an ion selective electrode. The molecular stacking properties of such salts are known but the particular impurity or dope introduced into the crystal lattice has been found to lead to the mobilization of charges which is necessary in order that the crystals may properly act in an ion selective electrode. When the electrode is placed in a solution containing the ion of the metal used in making the fatty acid salt, the crystal displays a change in electric potential through the lattice which may be measured.

It has been discovered that the proper dope or im- 3,649,568 Patented Mar. 14, 1972 purity to be used with each of these bivalent alkaline earth metal ions is another metal ion whose valence is other than two and which has an ionic radius, measured in angstroms (A.), which is close by less than .15 angstrom to that of alkaline earth metal ion it is associated with as expressed in the following table:

Major cation in crystal and its Minor cation (dope) and its Electrode specific forionic radius (A.) ionic radius (A.)

Bc-l--i- Be++ (r=0.31) Si (r=0.4l) Ba-i--l- Ba++ (r=1.43) Tl+ (r=l.49)

Ag+ (r=l.26) Sr++ Sr++ (r=1.13) or La+3 (r=1.15) Ca|+ Ca++ (r=0.99) Na+ (r=0.93) Mg++ Mg++ (r=0.66) Li-l- (r=0.68)

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS In making a crystal according to the invention so as to be suitable for use in an electrode selective with respect to magnesium cations, a magnesium stearate salt is prepared using stearic acid (CH3(CH2)16'COOH) whose melting point is 6870, chemically pure lithium chloride (LiCl), chemically pure magnesium chloride (MgCl2), and reagent grade acetone (CH3CO-CH3).

A 2.84 gram portion of stearic acid is dissolved in 70 millilitres of acetone. Then a mixture of 0.92 gram MgCl2 and 0.00142 gram of LiCl are dissolved in 2 millilitres of distilled water and the two solutions are mixed. A white precipitate of magnesium stearate, doped with the lithium, appears in the acetonic solution. This precipitate is then filtered and lwashed several times with acetone and then dried overnight at 60.

This dried stearate is then formed into a large single crystal using a single-pull Zone refinement apparatus, more fully described by Tensmeyer, Landis and Marshall in vol. 32 Journal of Organic Chemistry, p. 2901 (1967).

Briefly, this procedure involves heating the stearate in a controlled temperature container to melt at about C. Referring to FIG. 2, such a container 10 is shown heated by a coil 11 connected to a radio frequency generator, not shown, the connections 12 passing through an insulator 13 in the wall of a chamber 14 full of an inert gas, such as argon, pumped in through the opening 15.

An automatic lowering and lifting device indicated by the arrows 16 is adapted to lift a glass support rod 17 away from the container 10. Supported on rod 17 is a metal rod or stainless steel screw 18 with a magnesium stearate seedling crystal 19 attached to its lower end.

Screw 18 is iirst lowered until the seedling is brought into contact with the stearate melt 20 as indicated Iby the liquid level at 21. The automatic lifting device then is started and the screw 18 is lifted at a very slow rate, approximately one centimeter per hour. The lifting device also, at the same time, slowly turns or rotates screw 18 and the attached crystal at a rate as slow as 10 revolutions per minute. As the screw 18 is raised the large single crystal 22 grows as a rod having a peaked top at the 3 seedling 19, the crystal growth taking place at the bottom of the crystal rod as indicated by the liquid level at 23.

In this manner incoming molecules have time to orient themselves in the crystal lattice. The melt contains a large proportion of magnesium stearate molecules and a minute proportion of lithium, or dope, molecules. In these long-chain fatty acid molecules, the hydrocarbon portion of each stearate molecule has a strong ainity for the hydrocarbon tail of another stearate molecule and so, apparently, arrange themselves in the manner shown in PIG. 1 Where the hydrocarbon portion is shown as a rod, as at 25.

The remaining portion of the magnesium stearate molecules are arranged around the bivalent magnesium ion at 26. Since the lithium ion has approximately the same ionic radius, it apparently finds it easy to arrange itself in the lattice taking the place of a magnesium ion, as shown at 27.

Referring to FIG. 3, the strong attraction of the hydrocarbon portions provides for an orderly arrangement of the molecule, the magnesium stearate molecules being indicated at 28 and the lithium stearate molecules being indicated at 29.

The presence of the monovalent lithium ion causes an imbalance of the charges distribution at the point indicated at 30 in FIG. l, although it does not disturb the overall spacial arrangement of the crystal lattice as shown in FIG. 3. The site which is occupied by the Li+ ion will possess a net excess of negative charge. The electron probably is partially neutralized by the movement of Mg+| from the nearest neighbor. Similar to the jumping of holes in inorganic semiconductors, the imbalance of charges contributes to the mobility of Mg-i--iions in the crystal which, in turn, gives rise to a measurable E.M.F.

When the crystal Z2 has grown to the desired size, it may be removed and split to a wafer shape 32 by breaking along the lines of cleavage, as shown in FIG. 4.

To make the wet-type electrode A, a glass tube 33, threaded at its lower end, has a Teflon cap 34 screwed thereon, the cap having a cross-sectionally T-shaped passage 35 therethrough. The crystal wafer 32 is placed in the cross-arm portion of passage 35 and cemented therein at 36 by suitable adhesive.

In use, the tube 33 is partially filled with a magnesium chloride solution (MgCl2) at 37, for the transport of the Mg++ ions, and a silver-silver chloride reference electrode 38 is suspended therein by means not shown. The magnesium chloride solution has a known concentration of Mg-i--lions therein, 10-3 M, for example.

The specimen solution 40, to be tested, is in a container 41 and the electrode A can be placed directly therein, as shown. For comparison a conventional saturated calomel electrode B is also placed in the solution. Electrode B is usually in the form of a test tube 42 which has a hole 43 or holes in its bottom. It has a solution of calomel (HgClz-HgCl) and potassium chloride (KCl) therein, the hole 43 fbeing covered by crushed glass at 44 to prevent entry of the test solution and has another silver-silver chloride electrode 45 suspended therein. The two silver-silver chloride electrodes are connected to the two posts of a high input impedance electrometer 46 so as to obtain an reading.

The difference in concentration of Mg++ ions between the test solution and the internal filling solution of magnesium chloride 37 gives rise to a change in E.M.F. which can be amplified and measured lby the high input impedance electrometer 46. The alkaline earth stearate is very insoluble in aqueous media and, theoretically, the detection limit of Mg++ ion will be as low as the solubility product constant of magnesium stearate. The practical working range, however, is 10-5 M to l M of the Mg++ ion.

Preliminary testing with calcium stearate doped with sodium stearate indicates that the stearate crystals so formed act as an electrode for detecting the Ca+| ions when used in a wet electrode system such as described in connection with FIG. 4. Calcium chloride and sodium chloride were mixed with stearic acid dissolved in acetone, the amount of water to dissolve the chlorides being kept to the minimum. The proportion of sodium chloride to calcium chloride was about 200 to 1 and the proportion of chlorides to stearic acid was about 1 to 3 by weight. In the wet electrode the internal filling solution was a calcium chloride solution.

Similar testing with barium using a titanium salt as dope indicates that doped barium stearate crystals act as a specific electrode to detect and measure the concentration of the Ba-}+ ion in solution. The barium and titanium were mixed with the stearic acid solution in the form of chlorides and the proportions were roughly the same as for the calcium stearate mix. Barium chloride was used as the filling solution of the wet electrode.

Due to the scarcity of beryllium (rf-:0.31) and strontium (r=l.l3) in nature, no testing has yet been done for electrodes specific for these metals. The indications are, however, that suitable dopes for these metals are silicon, Si+4, (r=0.4l) and silver, Ag-}, (r=1.26) or lanthanum, La+3, (r=1.l5), respectively.

Also, due to the presence of lauric acid and palmitic acid (CH3-(CH2M4-COOH) in almost all commercially available stearic acid, it is -believed that these long-chain fatty acids are also capable of combining with the alkaline learth metals to form crystalline material for cation detection.

I claim:

1. A crystal suitable for use in a wet electrode for testing for and measuring the cation concentration in a solution containing a speciiied 1bivalent metal selected from the group consisting of calcium, magnesium and barium, comprising: the speci-tied metal salt of a long chain fatty acid selected from the group consisting of stearic acid, lauric acid and palmitic acid, doped with a minute portion of another metal which has a val-ence other than two and whose ion has an ionic radius differing from that of the specied metal ion by less than .l5 angstrom.

2. A crystal suitable tor use in a -wet electrode for testing for and measuring the cation concentration in a solution containing a specified bivalent metal selected from the lgroup consisting of calcium, magnesium and lbarium, comprising: the specified metal salt of stearic acid doped with a portion of another metal which has a valence other than two and whose ion has an ionic radius diering from that of the speciled metal ion lby less than .l5 angstrom, the portion of the other metal being less than 1/00 by weight of that of the speciiied metal.

References Cited UNITED STATES PATENTS 2,945,051 7/1960 Davis 26413 3,476,786 11/1969 Lally et al. 260-413 DOUGLAS l, DRUMMOND, Primary Examiner U.S. Cl. X.R. 26o-413; 317-237 

